Why mixed-airline trips fail differently
A bag can be valid on the outbound airline and fail on the return, feeder or codeshare-operated leg. The difficult part is not finding one airline's limit; it is finding the intersection of all the allowances you actually bought.
Use the operating airline, not just the ticket brand
For a codeshare, the marketing airline on the ticket may differ from the carrier operating the aircraft. BagFit asks you to select the operating airline for each leg because that is the rule set you should verify before departure.
Fare entitlement is separate from physical size
A 55 cm bag may physically fit an airline's overhead limit but still be excluded from a basic fare. This tool checks the pieces included in the fare option you select, not just the largest published aircraft-cabin dimension.